Thin film solid oxide fuel cell and method for forming

ABSTRACT

A thin film solid oxide fuel cell (TFSOFC) having a porous metallic anode and a porous cathode is provided. The fuel cell is formed by using a continuous metal foil as a substrate to epitaxially deposit a thin film electrolyte on one surface of the foil. The metal foil may then be made porous by photolithographically patterning and etching the other surface of the foil to form holes extending through the foil to the electrolyte/foil interface. The cathode is then formed on the electrolyte by depositing a second thin film using known film deposition techniques. Further processing may be used to increase the porosity of the electrodes. The metal foil may be treated before film deposition to have an atomically ordered surface, which makes possible an atomically ordered thin film electrolyte.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a Continuation-in-Part Application of, andclaims benefit of, U.S. patent application Ser. No. 09/534,385, whichwas filed Mar. 24, 2000, and which will issue as U.S. Pat. No. 6,645,656on Nov. 11, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention pertains to electrochemical devices such assolid oxide fuel cells (SOFCs) or ceramic fuel cells, particularly thinfilm solid oxide fuel cells (TFSOFCs). More particularly, a porousmetallic anode and a thin film conducting oxide porous cathode areprovided, along with methods for forming the electrodes and a thin filmelectrolyte.

[0004] 2. Description of Related Art

[0005] Fuel cells are energy-converting devices that use an oxidizer(e.g. oxygen in air) to convert the chemical energy in fuel (e.g.hydrogen) into electricity. A SOFC (also called a “ceramic fuel cell”)generally comprises a solid electrolyte layer with an oxidizer electrode(cathode) on one side of the electrolyte and a fuel electrode (anode) onthe other side. The electrodes are required to be porous, or at leastpermeable to oxidizer at the cathode and fuel at the anode, while theelectrolyte layer is required to be dense so as to prevent leakage ofgas across the layer. A TFSOFC has a thin electrolyte layer, on theorder of 1-10 micrometers thick, as described, for example, in U.S. Pat.No. 5,753,385. This reduces the ohmic resistance of the electrolyte andincreases the power density of the fuel cell. Because of the lowelectrolyte resistance, the TFSOFC can operate at lower temperatures.This increases the reliability and allows wider choices of materials forTFSOFC applications. Using the TFSOFC design can also reduce materialscosts and reduce the volume and mass of the fuel cell for a given poweroutput.

[0006] U.S. Pat. No. 5,753,385 discloses physical and chemicaldeposition techniques to synthesize the basic components of a TFSOFC. Inone technique, the electrodes are formed from ceramic powders sputtercoated with an appropriate metal and sintered to a porous compact. Theelectrolyte is formed by reactive magnetron deposition. Theelectrolyte-electrode interface is formed by chemical vapor depositionof zirconia compounds onto the porous electrodes.

[0007] U.S. Pat. No. 5,656,387 discloses an improved nickel andyttrium-stabilized zirconia (YSZ) anode and a method for making by DCmagnetron sputtering. The films were deposited on a surface ofyttria-stabilized zirconia (YSZ).

[0008] U.S. Pat. No. 5,106,654 discloses a method for matching thermalcoefficients of expansion in fuel cell or other electrochemical devices.A tubular configuration not employing thin films is described.

[0009] YSZ thin film fuel cells have generally been formed by depositingthe YSZ on a substrate that is not crystallographically ordered.Therefore, the YSZ is not ordered and thicker layers must be depositedto form a layer impermeable to gas. Michibata et al (“Preparation ofStabilized Zirconia Electrolyte Films by Vacuum Evaporation,” DenkiKagaku, 58, No. 11 (1990) demonstrated growth of dense but notatomically ordered YSZ films on nickel foil. They also provided nomechanism for increasing gas permeability of the nickel foil and claimedvery low maximum power output (7 mW/cm²) of a resulting fuel cell.

[0010] To make thin film solid oxide fuel cells more efficient and lessexpensive to fabricate, improved methods for forming the porouselectrodes and the non-porous electrolyte used in such devices areneeded. The electrolyte should be defect-free to avoid charge and gasleakage across the cell, and thin to provide lower electrical resistanceat moderate temperatures. Interconnect layers to make possible stackingof cells should be provided.

SUMMARY OF THE INVENTION

[0011] A method for forming a thin film solid oxide fuel cell (TFSOFC)with a porous metallic anode and an oxidizer-permeable cathode onopposite surfaces of a dense electrolyte layer is provided. Theelectrolyte layer may have an ordered crystal structure.

[0012] The fabrication process uses a thin dense metallic material suchas nickel foil as a substrate on which to grow the electrolyte. Thenickel foil may be appropriately rolled or otherwise processed toproduce an ordered crystal structure that allows the electrolyte layer,epitaxially grown on the nickel substrate, to be crystallographicallyordered. The nickel foil is later used as the anode after it is madeporous by lithographic patterning and etching by chemical or physicalprocesses.

[0013] Thin film oxide deposition technologies such as pulsed laserdeposition (PLD) or metal organic chemical vapor deposition (MOCVD) canbe used for the deposition of the oxide electrolyte as well as for theconducting oxide cathode. PLD is an ideal vehicle to develop very thinfilms for TFSOFC applications, while MOCVD is good for large area thinfilm fabrication. Sputtering, evaporation sol-gel, metal organicdeposition (MOD), electron-beam evaporation, chemical vapor deposition(CVD), molecular beam epitaxy (MBE), or other oxide film depositiontechniques can also be used. Because the substrate is dense and notporous, and in foil form, a dense electrolyte layer is easily depositedon it, and the difficulty of forming a dense, uniform electrolyte layeron a porous substrate is avoided. Also, because the solid metalsubstrate is used as a support, the electrolyte layer can be very thin.In addition, since the substrate is a continuous foil and can be madeatomically ordered, electrolyte film with ordered crystal structure canbe grown on appropriately prepared metallic foil substrates such asnickel.

[0014] Chemical or physical etching or a physical process such as laserdrilling may be used to fabricate pores in the metallic substrate (whichwill become the anode) after deposition of the electrolyte. The cathodelayer can be deposited on the opposite side of the electrolyte layer,either before or after etching or physical drilling of the metallicsubstrate. The cathode is usually a conducting oxide layer, which can bedeposited by PLD, MOCVD or other suitable oxide film depositiontechnique, thus forming the TFSOFC. A mixed ionic and electronicconductor film between the anode and the electrolyte may be deposited toenhance the activity of the porous anode structure. Stacked cells may beepitaxially grown using a substrate having an atomically orderedsurface.

DESCRIPTION OF THE DRAWINGS

[0015] For a more complete understanding of the present invention andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings in whichlike reference numbers indicate like features and wherein:

[0016]FIG. 1 is a schematic representation of the SOFC process FIG. 2 isa schematic representation of a solid oxide fuel cell with a porousmetallic electrode.

[0017]FIG. 3 illustrates the patterning process for the metallic anode.

[0018]FIG. 4 shows an SEM micrograph of a patterned and etched nickelanode.

[0019]FIG. 5 shows an SEM micrograph of LSCO films on YSZ developed byPLD: a) a dense film; b) a porous film.

[0020]FIG. 6 shows the x-ray diffraction pole-figure and phi-scan of the(111) peak of YSZ deposited on roll-textured nickel foil indicatingatomically ordered YSZ grown on roll-textured nickel foil.

[0021]FIG. 7 shows the x-ray diffraction pole-figures and phi-scans ofthe (111) peaks of YSZ and of Sm-doped CeO₂ for a YSZ/Sm—CeO₂ multilayerstructure grown on roll-textured nickel foil indicating atomicallyordered doped CeO₂ and subsequently grown atomically ordered YSZ.

[0022]FIG. 8 illustrates a mixed ionic and electronic conductor filmbetween the anode and electrolyte films.

[0023]FIG. 9 shows measured values of power output from a fuel cellformed according to methods disclosed herein.

[0024]FIG. 10 shows measured values of power output at differenttemperatures from a fuel cell formed according to methods disclosedherein.

DETAILED DESCRIPTION OF THE INVENTION

[0025] It is preferred to fabricate a TFSOFC with a thin electrolytelayer to reduce the resistive loss in the electrolyte. The electrolytelayer should be dense and pore-free to prevent gas leakage through it.It is also preferred that porous electrodes be used to increase the gastransport rate. These requirements increase the difficulty in thefabrication of TFSOFCs. We disclose herein an epitaxial film growthmethod to make a TFSOFC with the combined structure of a dense thin filmelectrolyte with porous or gas-permeable electrodes. As demonstrated inFIG. 1(a), the method first uses a thin dense metallic material 10, suchas nickel foil, as a substrate for the cell fabrication and as the cellanode. A dense but thin electrolyte layer 12 is then deposited on thesubstrate, as shown in FIG. 1(b), after which substrate 10 is madeporous by lithographic patterning and etching, resulting in pores 14(FIG. 1(c)) developed in the substrate but not in the electrolyte.Referring to FIG. 1(d), cathode 16 of the SOFC is deposited in thin filmform on electrolyte 12, either before or after etching of anode 10.Further processing can be used to improve the permeability andperformance of the electrodes, as disclosed below. The SOFCs fabricatedby this method can then be packaged into stacks, as indicated in FIG.1(e), where interconnects 18 couple the SOFCs.

[0026] A schematic example of a SOFC fabricated with the method providedhere is shown in FIG. 2. Metal substrate and anode 10 has been used inan epitaxial deposition process to form electrolyte layer 12. Cathode 16has been formed on electrolyte 12. Pores 14 are fabricated in metalsubstrate 10 by etching or physical drilling.

[0027] A preferred embodiment of a process for making a thin film solidoxide fuel cell is described as follows. It should be understood thatthe description of a preferred embodiment does not limit the scope ofthe methods and apparatus disclosed herein.

[0028] A nickel foil with a nominal thickness of 0.001 inch is used asthe substrate and the anode of the device. The substrate/anode thicknesscan be varied to accommodate a specific fuel cell design. Any metal oralloy that is stable under the operating temperature and reducingatmosphere at the anode can be used. The nickel foil can be also treatedto expose an atomically ordered surface by roll-texturing as describedby A. Goyal et al “High critical current density superconducting tapesby epitaxial deposition of YBa₂Cu₃O_(x) thick films on biaxiallytextured metals”, 69(12) Appl. Phys. Lett. 1795 (1996), or by Ion BeamAssisted Deposition (IBAD) as described by X. D. Wu et al, “High currentYBa₂Cu₃O_(7-δ) thick films on flexible nickel substrates with texturedbuffer layers”, 65(15) Appl. Phys. Lett. 1961 (1994), both of whicharticles are incorporated by reference herein. Such anatomically-ordered surface allows for the growth of an atomicallyordered electrolyte layer as well as subsequent layers such as thecathode and even the conducting interconnect layers that could act assubstrate for growth of stacked cells.

[0029] An electrolyte thin film oxide layer is deposited on thesubstrate/anode by, for example, PLD. Yttria stabilized zirconia (YSZ)is an example of a solid oxide electrolyte (although other solidelectrolytes may be used), and with PLD a target of YSZ is used todeposit a thin film of YSZ on the metallic substrate/anode. Purehydrogen or “forming gas” (4% hydrogen with 96% argon) or other reducinggas mixtures can be introduced into the thin film deposition chamber toreduce oxidization of the nickel or other metal substrate under any ofthe oxide thin film growth techniques. Other thin film oxides that canbe used as electrolytes include doped LaGaO₃, doped CeO₂ and multilayersof oxides such as YSZ/doped-CeO₂.

[0030] As an example, PLD using an excimer laser can be used for thedeposition of the electrolyte films. The thin film growth may becontinued to yield an electrolyte layer having a thickness in the rangefrom about 0.01 to about 10 micrometers (depending on the application).The electrolyte layer formed on a typical metal foil substrate generallyhas no long range atomic order. However, the electrolyte may bepreferentially atomically ordered with its (100) crystallographicdirection normal to the growth surface or with atomic order both normalto and in the plane of the growth surface (the surface of the substrate)by using an atomically ordered substrate. For example, usingroll-textured metal foil or IBAD-treated metal foil such as describedabove, the electrolyte can be grown with atomic order both normal to andin the plane of the growth surface.

[0031] The metallic substrate/electrolyte thin film structure may thenbe subjected to a patterning and etching procedure to fabricate pores inthe metallic anode. Photolithography is used to pattern areas of thesubstrate to be etched with a pore design defined by a mask. The maskthat is used as the template for the patterned pores can have variouspatterns in addition to just circular holes. The mask may have starpatterns, for example, or irregular shaped holes to increase theperimeter of the etched holes in the anode. The patterning process isshown in FIG. 3, where 10 indicates, for example, the nickel substrateand 12 indicates the YSZ thin film. One side or both sides of thestructure are coated with photo-resist 20, depending on materials used.A photo-resist film of several hundred to several thousand nanometersthickness is made on the nickel substrate by spin-coating photo-resiston the sample, using well known techniques. After baking thephoto-resist, mask 22 is put on the sample and the resist on the nickelor substrate side is exposed to UV light through the mask. The sample isthen put into a developer solution and the pattern is developed on thephoto-resist. After development, the sample is rinsed and the areashadowed by the mask is removed, while that exposed to UV light staysattached to the substrate. The sample is then post-baked to stabilizephoto-resist layer 20, which exhibits holes 24 developed in thephoto-resist.

[0032] Either a wet or dry etching process may be used. For wet etching,the complete structure is placed in an etching solution such as (but notrestricted to) ferric chloride for etching the metallic nickelsubstrate/anode and not the electrolyte. Other etching solutions couldalso be used for nickel or other metallic substrate etching. The thinfilm structure is maintained in the etching bath until the patternednickel area without photo-resist is etched through (approximately 180minutes at a temperature of 18° C. for a 0.001″ metallic nickelsubstrate foil, for example). Other times and temperatures may beselected. The photoresist is then removed with acetone, and aYSZ/porous-nickel structure is formed. FIG. 4 is the SEM micrograph of apatterned sample showing an array of holes through the nickel. The etchprocess is such that it leaves the oxide electrolyte intact, thus notresulting in gas leakage through the electrolyte, which would reduceperformance of the fuel cell. For physical formation of porosity, aprocess such as ion beam etching, reactive ion etching or laser drillingmay be used to pattern the anode. Again, holes should not be createdthrough the electrolyte layer.

[0033] A conducting oxide cathode thin film is then deposited on top ofthe oxide electrolyte (YSZ) to form the SOFC. ALa_(0.5)Sr_(0.5)CoO_(3-δ) (LSCO) thin film is used as the cathodematerial in this example, although other oxide thin film cathodematerials can also be used. The LSCO thin film cathode can also bedeposited by PLD, sputtering, MOCVD or other oxide depositiontechniques. FIG. 5a is an SEM micrograph of an LSCO cathode deposited onYSZ by PLD, showing that the LSCO film is dense and pore-free. There aresmall particles on the PLD-formed LSCO film, which are often seen onoxide films fabricated by PLD. This may be good for SOFC developmentsince it increases the surface area and could increase the reaction rateat the cathode. Details of PLD deposition of LSCO on YSZ are describedby Xin Chen, Naijuan Wu, Alex Ignatiev, Zuhua Zhang and Wei-Kan Chu in“Structure and Conducting Properties of La_(0.5)Sr_(0.5)CoO_(3-d) Filmson YSZ”, 350, Thin Solid Films, 130 (1999), which is incorporated byreference herein. For the case of an atomically ordered electrolyte,i.e. one formed on an atomically ordered substrate, the cathode can alsobe deposited under conditions such that it is also atomically ordered,i.e. it is grown epitaxially on the ordered electrolyte with, e.g., the(100) direction normal to the electrolyte surface and having additionalin-plane ordering.

[0034] Additional porosity may be desired in the cathode. The cathodecan be made more porous by specific processing as described by X. Chen,N. J. Wu, D. L. Ritums and A. Ignatiev in “Pulsed Laser Deposition ofConducting Porous La—Sr—Co—O Films”, 342, Thin Solid Films, 61-66(1999), which is incorporated by reference herein. To enhance LSCO filmporosity, the LSCO films can be deposited at room temperature onto theelectrolyte and then heated to high temperature for several hoursresulting in a porous columnar structure. The electrical conductivity ofthe LSCO is maintained in this process at a level of ˜10-3 ohm-cm orbetter. FIG. 5b is an SEM micrograph of a porous LSCO film fabricated onYSZ by PLD.

[0035] The above example shows one instance of application of the methoddisclosed herein for the fabrication of planar thin film solid oxidefuel cells. It can also be used in other designs such as tubular andmonolithic fuel cells. It also not only applies to SOFC's with a simpleelectrolyte layer, but also to those with complex electrolytes such as aYSZ/doped CeO₂ multi-layer electrolyte. It not only applies to SOFC's,but it also applies to the fabrication of other electrochemical deviceswith combined thin film electrolyte/porous electrode structure. The thinoxide films can be deposited not only as non-ordered polycrystallinefilms, but also as crystallographically ordered films.

[0036] N. Q. Minh has described in Science and Technology of CeramicFuel Cells, Elsevier Science B.V. (1995), that grain boundaries usuallyhave much lower ion conductivities than the bulk of an electrolytematerial. By developing crystallographically ordered thin electrolytefilms, grain boundaries can be reduced or even removed in theelectrolyte, thereby increasing ion conductivity and reducing theresistance of the electrolyte. Crystallographically-ordered films of YSZand CeO₂ have been deposited on metallic substrates using a uniquephoto-assisted MOCVD technique (PhAMOCVD) as described by A. Ignatiev,P. C. Chou, Q. Zhong, X. Zhang, and Y. M. Chen, “Photo-AssistedFabrication of YBCO Thick Films and Buffer Layers on Flexible Substratesfor Wire Applications”, 12, International Journal of Modern Physics B,3162 (1998), which is incorporated by reference herein. This techniquehas been used to fabricate highly ordered YSZ/CeO₂ multi-layers onroll-textured nickel foils, as demonstrated by the x-ray diffraction(XRD) data shown in FIGS. 6 and 7. FIG. 6 is an XRD pole-figure andphi-scan of the (111) peak of YSZ deposited by PhAMOCVD on roll-texturednickel foil. FIG. 7 is the XRD pole-figure and phi-scan of the (111)peak of YSZ and Sm doped CeO₂ from a YSZ/Sm—CeO₂ sample developed onroll-textured nickel foil with PLD. The full width at half max (FWHM) ofthe phi-scan peaks in both figures are only several degrees wide,indicating good crystalline ordering of the YSZ film on the metallicsubstrate and of the YSZ film grown on the Sm—CeO₂ layer. This epitaxialgrowth technique can be directly used for the SOFC development to testfor good crystalline ordering of the YSZ film, which is indicative ofthe desired high electrical conductivity of the electrolyte.

[0037] This disclosure is not limited to the examples of thin film oxidematerials cited above. For example, La_(1-x)Sr_(x)MnO₃, a widely usedcathode material for SOFCs, may be used with the methods disclosed here.It has high electrical conductivity, adequate chemical and structuralcompatibility with YSZ electrolytes, and an acceptable coefficient ofthermal expansion match with other SOFC components.La_(1-x)Sr_(x)Co_(1-y)Fe_(y)O_(3-δ) and La_(1-x)Sr_(x)Co_(1-y)Mn_(y)O₃are other cathode materials that may be used for SOFCs working atlowered temperature. Elements such as Y, Ca, Ba, Pr, Nd, Cu, and Ni canalso be used to replace elements in the cathode material in order tomodify the characteristics and performance of the cathode. Cathodescontaining In₂O₃ and RuO₂ can also be used as they may yield excellentelectrical conductivity for SOFC applications. The cathode can be formedbefore the etching of the anode or after the etching.

[0038] The electrolyte can also be varied by incorporating not only YSZbut also other electrolyte materials such as doped-CeO₂, stabilizedB₂O₃, perovskite oxide ion conductors such asLa_(1-x)Sr_(x)Ga_(1-y)Mg_(y)O₃ and doped BaCeO₃ and pyrochlore oxides(with the general formula A₂B₂O₇) such as Gd₂(Zr_(x)Ti_(1-x))₂O₇ andY₂(Zr_(x)Ti_(1-x))₂O₇. In addition, not only can single layers ofelectrolyte be used, but multi-layer and more complicated electrolytestructures can be formed. For example, doped CeO₂, when used as anelectrolyte, has very high ionic conductivity and also shows reducedovervoltage at the electrode/electrolyte interfaces. However, CeO₂ canbe reduced at low oxygen partial pressures to exhibit electronicconductivity. If a YSZ/doped-CeO₂ multi-layer structure is used for theelectrolyte, as in the example shown in FIG. 7, the ion conductivityadvantage of doped-CeO₂ is retained, while the electronic conductivityproblem can be mitigated by the insulating YSZ layer.

[0039] The anode can be fabricated not only from nickel, but may be madefrom other metals and alloys such as INCONEL, Haynes Alloy or HASTELLOY.In addition, the metal substrate can be treated with a buffer layer orlayers so as to better integrate with the thin film electrolyte. Thesebuffer layers may be of varying structure and composition, and willgenerally be used to match lattice parameters of the substrate andoverlayer for optimal epitaxial growth, to better match thermalexpansion coefficients of the substrate and overlayer to minimizethermal stress cracking, and to act as diffusion barriers to mitigateinterdiffusion of species from or to the substrate to or from theoverlayer, while still maintaining the fuel cell electrochemicalrequirements of reduced interface resistance and high ionicconductivity. Since the fuel-oxidizer activity for the YSZ/Nickel anodestructure is mostly limited to the three-phase boundary, the overallactivity of the porous anode structure fabricated by etching can beenhanced by increasing the area of the 3-phase boundary. This can bedone in several ways including the already noted patterning ofhigh-perimeter pores in the anode, the addition of a layer of porousanode material such as a nickel-YSZ cermet onto the anode after etching,or the deposition of a mixed ionic and electronic conductor film betweenthe anode and the electrolyte. Such a mixed ionic/electronic conductingfilm is shown at 11, FIG. 8. The film will not limit anode activity onlyto the three-phase boundary, but can extend it to the whole exposedmixed conductor surface. For example, doped-CeO₂ can be reduced to showmixed conducting character and can be used for the purpose of formingfilm 11. To exhibit mixed conductor behavior, the doped CeO₂ film can begrown under highly reduced conditions. CeO₂ can also be reduced bychanging its doping level and/or dopant material. Another possibleenhancement at the anode is to use a hydrogen-conducting metal or alloysuch as palladium or palladium alloy as the anode for a proton typeSOFC.

[0040] The anode formation method disclosed herein is different fromexisting techniques that form a porous anode on an electrolyte surfacein that it uses the metallic anode foil both as substrate and support.As a result, the electrolyte layer can be made very thin, avoiding theself-supporting problem for a thin electrolyte layer in SOFCfabrication.

[0041] Any interconnect material can be used for the SOFC designdisclosed here as long it is stable to both the oxidizing and reducingenvironments of the fuel cell. For example, LaCrO₃ is the most usedinterconnect material for SOFCs. Other interconnect materials includeoxides such as doped LaCrO₃, doped CoCr₂O₄ and doped YCrO₃, metals andalloys such as nickel, chromium, INCONEL, and chromium-iron alloys, andoxide-dispersed or coated metals and alloys. Metals and alloys are ofgreat interest for use as an interconnect material in the TFSOFCdisclosed here because they may be the same material as the anodedisclosed here (e.g., as described in U.S. Pat. No. 5,106,654, wherenickel foil is used as an interconnect for an SOFC) and they may befabricated at the same time as the anode. Metals and alloys can be madeoxidation-resistant and hence can work well at SOFC workingtemperatures, especially for low temperature SOFCs. Oxide dispersion andoxide coating can improve the oxidation resistance and strength of themetals and alloys, and also improve the thermal match with other SOFCcomponents. In addition, when these metals and alloys are the same asthe anode material, the SOFC fabrication can be greatly simplified. Theinterconnect material can be deposited using thin film depositiontechniques disclosed here for depositing other films in the SOFC. Forexample, PLD, MOCVD, sputtering, evaporation or chemical deposition maybe used. These films can then be patterned to provide the flow channelsneeded for fuel and oxidizer transport in the interconnect region.

[0042] The commonly used stack designs can be used with the materialsand methods disclosed here. For example, the methods disclosed here canbe used to fabricate SOFC stacks with tubular, segmented-cell-in-series,monolithic, and flat-plate designs. Monolithic, and flat-plate designare of the greatest interest with the systems disclosed here, becausethe SOFCs can be made in a planar shape, which is good for forming aporous anode by patterning and etching using photolithography. A tubularfuel cell can be fabricated on a tubular metal substrate or on a flatmetal foil, which is flexible and can be bent into tubular form afterthe film deposition processes.

[0043] A fuel cell was fabricated by the methods described above andoutlined in FIG. 1. The data were obtained for a fuel cell incorporatinga patterned atomically textured nickel anode, an atomically ordered YSZelectrolyte layer and an atomically ordered LSCO cathode layer. The cellhad a total area of 0.25 cm². The electrodes and electrolyte were formedby the PLD process. The fuel cell was supplied hydrogen and oxygen andelectrical current and voltage output were measured. Results are shownin FIG. 9. Note that maximum power output was more than 600 mW/cm². Thisis a very satisfactory result. In comparison, for example, the fuel cellof Michibata referenced above produced a maximum of 7 mW/cm².

[0044] Another fuel cell was fabricated by the methods described above.The data were obtained for a fuel cell incorporating a polycrystallinenickel foil based porous anode, a polycrystalline YSZ electrolyte layerand a porous LSCO cathode layer. The cell had a total area of ˜0.1 cm².The electrodes and electrolyte were formed by the PLD process. The fuelcell was supplied hydrogen and oxygen and electrical current and voltageoutput were measured. Results are shown in FIG. 10 for differentoperation temperatures. Note that maximum power output of more than 100mW/cm² was obtained at the low operation temperatures of below 600° C.This is a very satisfactory result.

[0045] The foregoing disclosure and description are illustrative andexplanatory thereof, and various changes in the details of the methodand apparatus can be made without departing from the spirit of theinvention.

We claim:
 1. A method for forming a thin film solid oxide fuel cell, comprising: (a) supplying a thin continuous metal foil having a first and a second side; (b) depositing a thin film solid oxide layer on the second side of the metal foil to form a film of electrolyte, the film of electrolyte having a second side and an electrolyte/metal foil interface; (c) forming a plurality of holes extending through the first side of the metal foil to the electrolyte/metal foil interface to form a porous anode from the metal foil; and (d) depositing a thin film cathode on the second side of the film of electrolyte to form a thin film solid oxide fuel cell.
 2. The method of claim 1 wherein step (c) is performed before step (b).
 3. The method of claim 1 wherein step (d) is performed before step (c).
 4. The method of claim 1 wherein the thin continuous metal foil is treated to expose an atomically ordered surface on the second side of the metal foil before step (b).
 5. The method of claim 4 wherein the metal foil is treated by roll-texturing.
 6. The method of claim 4 wherein the metal foil is treated by Ion Beam Assisted Deposition (IBAD).
 7. The method of claim 1 wherein a buffer layer is applied to the second side of the metal foil before step (b).
 8. The method of claim 1 wherein the step of depositing a thin film solid oxide is performed using pulsed laser deposition (PLD).
 9. The method of claim 1 wherein the step of depositing a thin film solid oxide is performed using metal organic chemical vapor deposition (MOCVD).
 10. The method of claim 1 wherein the step of forming a plurality of holes through the metal foil is performed by photolithography followed by etching.
 11. The method of claim 1 wherein the step of forming a plurality of holes through the metal foil is performed by a physical process selected from the processes of laser drilling, ion beam etching and reactive ion etching.
 12. The method of claim 1 further comprising the step of determining atomic order of the films deposited in step (b) or (d) by x-ray diffraction measurements.
 13. The method of claim 1 wherein the metal foil is hydrogen-permeable and at least a part of the plurality of holes formed in step (c) do not extend to the electrolyte/metal foil interface.
 14. The method of claim 1 further comprising the step of depositing a plurality of layers in step (b).
 15. The method of claim 1 wherein in step (d) the cathode is deposited at a first temperature and further comprising the step of increasing the temperature of the cathode to a temperature higher than the first temperature so as to form a porous columnar structure in the cathode.
 16. A method for forming a stack of thin film solid oxide fuel cells, comprising: supplying a plurality of thin film solid oxide fuel cells formed according to claim 1; and interconnecting the fuel cells by depositing a layer of interconnecting material between an anode of a first fuel cell and a cathode of a second fuel cell, the layer having channels for transport of fuel and oxidizer to the first and second fuel cells.
 17. The method of claim 15 wherein the step of depositing is performed by pulsed laser deposition, metal organic chemical vapor deposition, sputtering, evaporation or chemical deposition. 